Biometric unlock stopped working. What can I do?

You are never locked out. The lock screen always offers a fallback: "Unlock with PIN", or "Sign in with recovery phrase". The biometric is a local gate, not an encryption key, so your notes are untouched either way.

Then fix the gate. Enrollment is per-device, so a new phone, a fresh browser profile, or a reinstall needs re-enrolling: Settings > Security > "Biometric Lock", disable, then "Enable biometric unlock" again. It also requires "Trust this device" (untrusted sessions clear when the tab closes) and hardware with a platform authenticator (Touch ID, Face ID, Windows Hello). If a password manager like Bitwarden or 1Password pops up instead of the system prompt, dismiss it and retry, or turn off its passkey capture for the site - the app requests the built-in authenticator, but some managers intercept anyway.